Rewrite version result to fix column name (#179)

* Rewrite version result to fix column name

When `select version()` is run, return a column named `version` instead
of `version()` so we are more compatible with postgresql behaviour.

This is relied upon by some clients, including pgadmin4.

* Fix clippy warning

Author: mjgarton

datafusion-postgres/src/sql/parser.rs

@@ -7,6 +7,8 @@ use datafusion::sql::sqlparser::parser::ParserError;
 use datafusion::sql::sqlparser::tokenizer::Token;
 use datafusion::sql::sqlparser::tokenizer::TokenWithSpan;
 
+use crate::sql::rules::FixVersionColumnName;
+
 use super::rules::AliasDuplicatedProjectionRewrite;
 use super::rules::CurrentUserVariableToSessionUserFunctionCall;
 use super::rules::FixArrayLiteral;
@@ -212,6 +214,7 @@ impl PostgresCompatibilityParser {
                 Arc::new(CurrentUserVariableToSessionUserFunctionCall),
                 Arc::new(FixCollate),
                 Arc::new(RemoveSubqueryFromProjection),
+                Arc::new(FixVersionColumnName),
             ],
         }
     }

datafusion-postgres/src/sql/rules.rs

@@ -770,6 +770,50 @@ impl SqlStatementRewriteRule for RemoveSubqueryFromProjection {
     }
 }
 
+/// `select version()` should return column named `version` not `version()`
+#[derive(Debug)]
+pub struct FixVersionColumnName;
+
+struct FixVersionColumnNameVisitor;
+
+impl VisitorMut for FixVersionColumnNameVisitor {
+    type Break = ();
+
+    fn pre_visit_query(&mut self, query: &mut Query) -> ControlFlow<Self::Break> {
+        if let SetExpr::Select(select) = query.body.as_mut() {
+            for projection in &mut select.projection {
+                if let SelectItem::UnnamedExpr(Expr::Function(f)) = projection {
+                    if f.name.0.len() == 1 {
+                        if let ObjectNamePart::Identifier(part) = &f.name.0[0] {
+                            if part.value == "version" {
+                                if let FunctionArguments::List(args) = &f.args {
+                                    if args.args.is_empty() {
+                                        *projection = SelectItem::ExprWithAlias {
+                                            expr: Expr::Function(f.clone()),
+                                            alias: Ident::new("version"),
+                                        }
+                                    }
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+        }
+
+        ControlFlow::Continue(())
+    }
+}
+
+impl SqlStatementRewriteRule for FixVersionColumnName {
+    fn rewrite(&self, mut s: Statement) -> Statement {
+        let mut visitor = FixVersionColumnNameVisitor;
+        let _ = s.visit(&mut visitor);
+
+        s
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -1021,4 +1065,16 @@ mod tests {
             "SELECT a.attname, pg_catalog.format_type(a.atttypid, a.atttypmod), (SELECT pg_catalog.pg_get_expr(d.adbin, d.adrelid, true) FROM pg_catalog.pg_attrdef d WHERE d.adrelid = a.attrelid AND d.adnum = a.attnum AND a.atthasdef), a.attnotnull, (SELECT c.collname FROM pg_catalog.pg_collation c, pg_catalog.pg_type t WHERE c.oid = a.attcollation AND t.oid = a.atttypid AND a.attcollation <> t.typcollation LIMIT 1) AS attcollation, a.attidentity, a.attgenerated FROM pg_catalog.pg_attribute a WHERE a.attrelid = '16384' AND a.attnum > 0 AND NOT a.attisdropped ORDER BY a.attnum;",
             "SELECT a.attname, pg_catalog.format_type(a.atttypid, a.atttypmod), NULL, a.attnotnull, NULL AS attcollation, a.attidentity, a.attgenerated FROM pg_catalog.pg_attribute AS a WHERE a.attrelid = '16384' AND a.attnum > 0 AND NOT a.attisdropped ORDER BY a.attnum");
     }
+
+    #[test]
+    fn test_version_rewrite() {
+        let rules: Vec<Arc<dyn SqlStatementRewriteRule>> = vec![Arc::new(FixVersionColumnName)];
+
+        assert_rewrite!(&rules, "SELECT version()", "SELECT version() AS version");
+
+        // Make sure we don't rewrite things we should leave alone
+        assert_rewrite!(&rules, "SELECT version() as foo", "SELECT version() AS foo");
+        assert_rewrite!(&rules, "SELECT version(foo)", "SELECT version(foo)");
+        assert_rewrite!(&rules, "SELECT foo.version()", "SELECT foo.version()");
+    }
 }